Matthew Shields flashes a smile and high-fives Mason Wilde with the prosthetic on his right hand. Born without fingers on that hand, Matthew, 9, now uses his Robohand to open doors, carry books and catch a ball—thanks to Mason, 17, who made the device with a 3-D printer at the Johnson County Library in Overland Park, Kan.

“It definitely made me proud,” says Mason, a junior at Louisburg (Kan.) High School. Matthew’s mother, Jennifer Shields, noticed last fall that her son’s birth defect was making the third-grader self-conscious and affecting him socially. But even with health insurance, the single mother knew she couldn’t afford a professionally made prosthetic.

Researching online, Jennifer found Robohand, the mechanical hand invented by South African carpenter Richard van As, who lost four fingers in a circular saw accident, and theatrical props maker Ivan Owen, in Bellingham, Wash. The pair posted the free digital design last year on thingiverse.com. “I looked at the plans, but had no idea how to do it,” recalls Jennifer, 43.

Mason, however, eagerly accepted the challenge. A straight-A student who aspires to be an engineer, he previously had read about three-dimensional printer technology. “I downloaded all the files and spent about three hours scaling the hand to fit Matthew,” Mason says.

Feeding and melting plastic filament to make parts on the 3-D printer took about eight hours. Mason assembled the 20 pieces with nylon cord and stainless steel screws. He attached the mechanical hand to a glove-like cuff that he molded from thermoplastic to fit Matthew’s hand. The materials cost about $60.

“It’s awesome,” says Matthew about his Robohand. “I could climb a gigantic mountain. Maybe I could clean my room faster.” Matthew uses backward and forward movements of his wrist to make the fingers open and close. The mechanical hand attracts the right kind of attention, Jennifer says. “Instead of, ‘What happened to your hand?’ it’s, ‘Hey, that’s cool,’” she says. “We’re very grateful to Mason.”

Meanwhile, Mason has established a nonprofit organization to raise money to buy a 3-D printer and make prosthetics for other people. “It’s an amazing feeling to be able to give someone a hand,” he says, “something we often take for granted.”'